The complexity of autoimmune disease calls for complementary approaches of intervention. This can only be attained by an increase in understanding the molecular mechanisms that elicit disease. The signal transducer and activator of transcription 3 (STAT3) plays a pivotal role in both acute and chronic inflammatory responses that contribute to the tissue damage that occurs in autoimmunity. The long-term goal is to elucidate mechanisms that regulate STAT3 nuclear function in order to design targeted inhibitors. The specific objective of this focused proposal is to determine the protein-protein interface between STAT3 and its nuclear import adapter, importin-alpha3. To function as a transcription factor, STAT3 must gain entrance to the nucleus and for this reason nuclear trafficking is a potential target for intervention. Increasing evidence indicates that STAT3 is a central player in chronic inflammation that occurs in T cell mediated autoimmune diseases. STAT3 is necessary for the development of T helper 17 cells and is causally linked to its immune pathology. Cytokines like interleukin-6 activate STAT3 by tyrosine phosphorylation, enabling it to bind to specific DNA targets and induce gene expression. Therefore nuclear translocation is central to the role of STAT3 as a transcription factor. More importantly, nuclear import of STAT3 is continual and independent of tyrosine phosphorylation. The passage of large molecules between the cytoplasm and nucleus is restricted and is mediated by karyopherin proteins that act as import carriers. STAT3 nuclear import ifs facilitated by direct interaction with the importin-alpha3 adapter. However, the specific protein-protein interface of the STAT3-importin-alpha3 complex remains to be determined. To achieve this goal two specific aims will be pursued: Specific Aim #1: To express and purify large quantities of STAT3 and importin-alpha3 and to obtain three-dimensional crystals suitable for high resolution structure determination. Specific Aim #2: To solve the crystal structure of the STAT3-importin-alpha3 complex and to analyze the protein-protein interface. The knowledge gained with accomplishment of these aims is expected to contribute to the development of inhibitors that can be later translated to the clinic. PUBLIC HEALTH RELEVANCE: This proposed research is relevant to the mission of the National Institute of Allergy and Infectious Diseases because it will provide fundamental knowledge to inhibit a transcription factor that functions in the progression of autoimmune diseases such as multiple sclerosis, inflammatory bowel disease, systemic lupus erythematosus, psoriasis, and rheumatoid arthritis. Achievement of the aims is expected to contribute to an understanding of the function of the STAT3 transcription factor, and to contribute to the development of clinically useful inhibitors.